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Derivatives and Hedging Activity - Summary of Amounts of Derivative Gains (Losses) Recognized In Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Earnings | [1] | $ (17.2) | $ 8.5
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Earnings into Earnings |  | (0.7) | 1.8
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing |  | 0.4 |
Commission revenue [Member] | Foreign Exchange Contracts [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Earnings | [1] | 3.7 | 4.3
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Earnings into Earnings |  | (0.1) | 0.7
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing |  | (0.2) |
Compensation expense [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Earnings into Earnings |  | (0.2) | 0.5
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing |  | 0.4 |
Operating expense [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Earnings into Earnings |  | (0.1) | 0.4
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing |  | 0.2 |
Interest expense [Member] | Interest Rate Contracts [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Earnings | [1] | (20.9) | 4.2
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Earnings into Earnings |  | (0.3) | $ 0.2
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing |  | $ 0.0 |
[1] | For the three-month period ended March 31, 2019, the amount excluded from the assessment of hedge effectiveness for our foreign exchange contracts recognized in Other comprehensive earnings was a loss of $0.1 million.

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